1. Field of the Invention
The present invention relates to a donor film and methods for fabricating patterned organic electroluminescent devices using the donor film, and more particularly to methods for fabricating patterned organic electroluminescent devices using a donor film for thin film transfer, in which at least one of an organic layer and a metal layer are transferred from an upper donor layer to a lower acceptor layer by at least one of light, heat, electric energy and pressure, the donor film containing a soft polymer film layer having a glass transition temperature of not greater room temperature.
2. Description of the Related Art
An organic electroluminescent (EL) device is a self-emission display using a phenomenon in which, when a current is applied to a fluorescent or phosphorescent organic compound film, light is emitted from the organic compound film by electron-hole recombination occurring. The organic EL device is lightweight, has non-complex components, and has a simplified manufacturing process. Furthermore, the organic EL device exhibits a high image quality and a wide viewing angle. Also, the organic EL device can present dynamic images and can achieve high color purity. In addition, the organic EL device has advantageous electrical features suitable for portable electronic devices, such as lower power consumption or lower voltage driving. The organic EL device can be used in various fields, including displays in electronic devices, backlight units, and so on.
Generally, the organic EL devices have a sequentially stacked structure of an anode, an organic layer including a hole transport layer, a light-emitting layer and an electron transport layer, and a cathode. In order to achieve more efficient injection of holes and electrons, a hole injection layer may further be provided between the anode and the hole transport layer, and an electron injection layer may further be provided between the electron transport layer and the cathode.
Here, the holes injected from the anode migrate to the light-emitting layer via the hole injection layer and the hole transport layer, and electrons from the cathode are injected into the light-emitting layer via the electron injection layer and the electron transport layer. The electrons and holes are recombined at the light-emitting layer to generate excitons. The generated excitons cause the light-emitting layer to emit light corresponding to an energy gap of the excitons deactivated from an excited state to a ground state, thus forming an image.
The anode is made of transparent conductive material having a high work function, such as ITO, IZO, or ITZO, and the cathode is made of a metallic material having a low work function and being chemically stable.
A passive matrix type organic EL device, which is one of organic EL devices, comprises an anode, a cathode, and a multi-layered organic layer interposed therebetween, and emits light at an intersection of the anode and the cathode when a current is applied between the anode and the cathode. Here, the anode is formed on the top of the organic layer in a predetermined pattern.
Meanwhile, in order to attain full-color organic EL devices, it is necessary to micro-pattern the organic layer including the light-emitting layer, the electron transport layer, and the hole transport layer.
One approach for micro-patterning the organic layer is a lithography technique. In the lithography technique, an organic layer is coated with photoresist, exposed and developed to obtain a photoresist pattern, and the obtained photoresist pattern is used in micro-patterning the organic layer. According to this approach, since the organic layer is liable to deform due to an organic solvent used during the process or developer residues, it is substantially impossible to put into practice. Another approach is a vacuum deposition method using a mask, which is, however, disadvantageous in that micro-patterning is difficult to achieve.
U.S. Pat. No. 5,937,272 discloses a method of forming an advanced patterned organic layer in a full color organic electroluminescent display device, wherein a donor support is coated with a transferable coating of an organic EL material. The donor support is heated to cause the transfer of the organic EL material onto the designated recessed surface portions of the substrate forming the colored EL medium in the designated subpixels. Optical masks and, alternatively, an aperture mask are used to selectively vapor deposit respective red, green, and blue organic EL media into the designated color EL subpixels. This method is similar to a thermal deposition technique in which an organic material is heated under vacuum and deposited through a shadow mask.
Another method is a laser induced thermal imaging process. In order to apply the laser induced thermal imaging process, a light source, a transfer film and a substrate are needed, and light coming out of the light source is absorbed by a photothermal conversion layer of the transfer film to be converted into heat energy so that a transfer layer forming material of the transfer film is transferred onto the substrate by heat energy to form a desired image.
Since it is necessary to convert laser induced light into heat energy, the photothermal conversion layer is made of a high thermal conversion capability, e.g., organic compounds such as carbon black or IR-pigments, metallic materials such as aluminum, oxides of the metallic materials, or mixtures of these materials.
U.S. Patent Published Application No. 2004-0191564 discloses a donor film of a low molecular weight full color organic electroluminescent display device, the donor film comprising: a substrate film; a photothermal conversion layer formed on the upper part of the substrate film; and a transfer layer formed on an upper part of the photothermal conversion layer and formed of a material comprising a low molecular weight material, wherein a part of the transfer layer which is irradiated and heated by a laser is separated from the photothermal conversion layer according to change of an adhesion force of the transfer layer with the photothermal conversion layer, wherein a part of the transfer layer which is not irradiated by the laser is fixed to the photothermal conversion layer by an adhesion force of the transfer layer with the photothermal conversion layer, and an adhesion force between a substrate of an organic electroluminescent display device to which the material comprising low molecular weight material formed on the transfer layer is transferred and the material comprising low molecular weight material and an adhesion force between the photothermal conversion layer and the material comprising low molecular weight material are greater than an adhesive force between the material comprising low molecular weight material of a laser irradiated region in the transfer layer and the material comprising low molecular weight material of a laser non-irradiated region, so that the material comprising the low molecular weight material of the laser irradiated region and the material comprising the low molecular weight material of the laser non-irradiated region are separated with respect to each other to cause mass transfer from the photothermal conversion layer to the substrate.
However, in the case where a pattern is transferred using the laser induced thermal imaging (LITI) method, since a donor film has poor flexibility and poor adhesion between the donor film and a lower acceptor film, a large amount of energy is required. In addition, since only organic materials are transferable, it is necessary to separately deposit a metal after patterning an organic layer to fabricate a passive matrix type organic EL device, which is quite burdensome. Further, this process requires a photothermal conversion layer.
According to a cold-welding method developed by Kim et. al. [Science, vol 288, p 831 (2000)], a desired portion of a metal layer coated on a substrate is stamped using a metal-metal adhesive force, and then removed. That is, organic layers are stacked on a substrate, and a cathode layer is deposited over the entire surface of the resulting structure. A stamp having a metal layer deposited thereon is pressed onto the cathode layer by applying a sufficient pressure thereto, and then the stamp is removed, thereby forming a cathode pattern.
In this case, however, since a large amount of pressure and a metal-metal adhesive force are used, this method is restrictively applied to metals having a high work function, or alloys of these metals, such as Au—Au, Ag—Ag, Pd—Pd. In addition, since this method is a lift-off method, rather than a transfer method, it is used only for patterning purposes after fabrication of a device.
Rhee and Lee developed a cathode transfer method, as disclosed in [Applied Physics Letters, vol. 81, p 4165, (2002)], including transferring pretreated and prepatterned metal on a substrate onto the surface of organic layers of the device by pressing, utilizing a difference in the adhesion strength of the metal between the substrate and the underlying organic layer. In this case, however, since sufficiently large amount of pressure is used and glass mold is used, the organic layer may be physically damaged and a release promotion layer is essentially needed.
Patterning methods such as micro contact printing, micro molding, nano transfer printing and so on use polydimethylsiloxane (PDMS), which is a kind of elastomeric silicone-based rubber and has a low glass transition temperature.
Korean Patent Publication No. 2003-0073578 discloses a passive matrix type organic EL device comprising a first electrode formed on a substrate in a first direction; an organic EL emitting layer formed on the entire surface of the substrate covering the first electrode; and a patterned second electrode positioned over the organic EL emitting layer in a second direction perpendicular to the first direction and by laser induced thermal imaging using a PDMS mold having an uneven portion with recessed regions and protruding regions repeated arranged to satisfy relationships W/L=0.2-20 and D≧L×20. In the disclosed method, the PDMS mold is attached to the second electrode and thermally cured to separate the second electrode from the protruding region of the mold, thereby completing the patterned second electrode. However, this method is basically the same as the cold-welding method in that the second electrode is separated using a difference in the relative adhesion force between the mold and the second electrode.